J. E. Marr and R. H. Aclie — The Lakes of Snoirdon. 59 



a tliin skin on the surface of the rock in many places. Rock, as he 

 states, occurs in the stream which comes from the lake at a distance 

 of forty or fifty feet below the exit, but the left bank of the stream 

 is bounded by drift for a long way beneath this, and the stream is in 

 places obviously cutting between drift and rock; nevertheless, we 

 do not believe that the old exit was here. Viewed from above, 

 a depression is seen running diagonally across the moraine-covered 

 ground between the two streams mentioned by Mr. Watts, and this 

 depression is marked by some pools, one of which is of sufficient size 

 to be inserted upon the six-inch map. The depression joins the 

 stream south-west of Llyn Teyrn, and the first live rock seen along 

 this line occurs between Clogwyn Aderynand Glogwyn Pen-llechen, 

 south of Llyn Teyrn, at a vertical distance of nearly 200 feet below 

 the level of Llyn Llydaw. The lesson taught by Llyn du'r Arddu 

 proves that a buried valley need not show any marked traces upon 

 the surface ; and we believe that, even though the greater part of 

 the moraine material forms a thin skin over the rock, a buried 

 channel runs in an easterly direction along the route we have 

 indicated. 



The water of Llyn Llydaw is described by Professor Ramsay as 

 being "of a green colour, like some of the lakes of Switzerland," 

 though the difference of colour between the w^aters of Glaslyn and 

 Llydaw did not appear to us to be very marked when we visited the 

 lakes in the early spi'ing. 



Above Llydaw lies Glaslyn, at a height of 1,970 feet above the 

 sea, and immediately below the great precipice surmounted by 

 Y Wyddfa, the highest point of Snowdon. The tarn is very in- 

 structive. It is, as Mr. Watts remarks, " bounded on all sides by 

 live rock, except at and near its outlet. This exit is over moraine, 

 which, however, is not very deep, for rock makes its appearance just 

 below, and in such a way as almost to compel belief in a complete rock- 

 bar. Besides the present course of the effluent stream is a parallel strip 

 of moraine running down towards Llyn Llydaw, but living rock soon 

 makes its appearance in this." This parallel slip of moraine looks 

 quite insignificant when viewed from the path ; but when visited 

 is found to be of considerable width. It joins the main stream at 

 a vertical height of at least 50 feet below the exit, and at the 

 junction a small stream is seen cutting its way backwards in the 

 drift. Between the exit and the junction of this . drift-filled 

 depression with the present stream is a waterfall, and the water has 

 here cut a mere groove in the rock. Moreover, we here meet with 

 a most significant feature : the bottom of the drift-filled depression is 

 at a lower level than the present stream, which runs at the side of the 

 valley, being separated from tl-ie lowest part by a low slielf of rock. 

 We here find a repetition of what one of us has previously noticed 

 in the case of the tarn Smallwater, near Haweswater in Westmore- 

 land (Quart. Journ. Geol. Soc, vol. xxi, p. 37), and we believe that 

 the explanation given in that case is applicable to Glaslyn also. 

 There is a feature of interest connected with the outline of the tarn. 

 A bay occurs on the north side, whose shore-line forms a curve 



